Non-volatile memory devices that may be electrically erased and programmed may retain data even if a supplied power is discontinued. A representative example of non-volatile memory devices is flash memory.
Memory cells that constitute flash memory may include a cell transistor having a control gate, a floating gate, a source and a drain. A cell transistor of a flash memory may be programmed or erased according to the Fowler-Nordheim (F-N) tunneling mechanism.
Data in a cell transistor may be erased by applying a ground voltage to a control gate of the cell transistor and applying a voltage higher than a power supply voltage to a semiconductor substrate (or bulk). Under such erasing bias conditions, a strong electric field may be formed between a floating gate and the semiconductor bulk due to a large voltage difference therebetween. Thus, electrons present in the floating gate may be discharged to the semiconductor bulk according to the F-N tunneling mechanism. In this case, the threshold voltage of the erased cell transistor may decrease.
The cell transistor may be programmed by applying a voltage higher than the power supply voltage to the control gate and applying a ground voltage to a drain and the semiconductor bulk. Under such bias conditions, electrons may be injected into the floating gate of the cell transistor according to the F-N tunneling mechanism. In this case, the threshold voltage of the programmed cell transistor may increase. A state in which electrons are injected into the floating gate may be referred to as a “programming state,” and a state in which electrons do not exist in the floating gate may be referred to as an “erased state.” A threshold voltage in the programming state may be greater than zero and a threshold voltage in the erased state may be less than zero.
Recently, research has been conducted into development of a multi-level flash memory capable of storing two or more data bits in one memory cell in order to improve the degree of intensity of flash memory. A memory cell that may store multiple data bits may be referred to as a multi-level cell (MLC), and a memory cell that may store a single data bit may be referred to as a single-level cell (SLC).
The MLC may have four or more threshold voltage distributions, and four or more data storing states corresponding to the distributions in order to store two or more data bits. However, as more data bits are stored in the MLC, more threshold voltage distributions may be needed by the MLC, thereby increasing a time required to read the data from the MLC.